forked from lijiext/lammps
400 lines
19 KiB
Plaintext
400 lines
19 KiB
Plaintext
"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
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:link(lws,http://lammps.sandia.gov)
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:link(ld,Manual.html)
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:link(lc,Section_commands.html#comm)
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:line
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fix srd command :h3
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[Syntax:]
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fix ID group-ID srd N groupbig-ID Tsrd hgrid seed keyword value ... :pre
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ID, group-ID are documented in "fix"_fix.html command
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srd = style name of this fix command
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N = reset SRD particle velocities every this many timesteps
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groupbig-ID = ID of group of large particles that SRDs interact with
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Tsrd = temperature of SRD particles (temperature units)
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hgrid = grid spacing for SRD grouping (distance units)
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seed = random # seed (positive integer) :ul
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zero or more keyword/value pairs may be appended :ulb,l
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keyword = {lamda} or {collision} or {overlap} or {inside} or {exact} or {radius} or {bounce} or {search} or {cubic} or {shift} or {tstat} or {rescale} :l
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{lamda} value = mean free path of SRD particles (distance units)
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{collision} value = {noslip} or {slip} = collision model
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{overlap} value = {yes} or {no} = whether big particles may overlap
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{inside} value = {error} or {warn} or {ignore} = how SRD particles which end up inside a big particle are treated
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{exact} value = {yes} or {no}
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{radius} value = rfactor = scale collision radius by this factor
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{bounce} value = Nbounce = max # of collisions an SRD particle can undergo in one timestep
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{search} value = sgrid = grid spacing for collision partner searching (distance units)
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{cubic} values = style tolerance
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style = {error} or {warn}
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tolerance = fractional difference allowed (0 <= tol <= 1)
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{shift} = {yes} or {no} or {possible} = SRD bin shifting for better statistics
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{yes} = perform bin shifting each time SRD velocities are rescaled
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{no} = no shifting
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{possible} = shift depending on mean free path and bin size
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{tstat} value = {yes} or {no} = thermostat SRD particles or not
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{rescale} value = {yes} or {no} or {rotate} or {collide} = rescaling of SRD velocities
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{yes} = rescale during velocity rotation and collisions
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{no} = no rescaling
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{rotate} = rescale during velocity rotation, but not collisions
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{collide} = rescale during collisions, but not velocity rotation :pre
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:ule
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[Examples:]
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fix 1 srd srd 10 big 1.0 0.25 482984
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fix 1 srd srd 10 big 0.5 0.25 482984 collision slip search 0.5 :pre
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[Description:]
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Treat a group of partilces as stochastic rotation dynamics (SRD)
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particles that serve as a background solvent when interacting with big
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(colloidal) particles in groupbig-ID. The SRD formalism is described
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in "(Hecht)"_#Hecht. The key idea behind using SRD particles as a
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cheap coarse-grained solvent is that SRD particles do not interact
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with each other, but only with the solute particles, which in LAMMPS
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can be spheroids, ellipsoids, or line segments, or triangles, or rigid
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bodies containing multiple spherioids or ellipsoids or line segments
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or triangles. The collision and rotation properties of the model
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imbue the SRD particles with fluid-like properties, including an
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effective viscosity. Thus simulations with large solute particles can
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be run more quickly, to measure solute propoerties like diffusivity
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and viscosity in a background fluid. The usual LAMMPS fixes for such
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simulations, such as "fix deform"_fix_deform.html, "fix
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viscosity"_fix_viscosity.html, and "fix nvt/sllod"_fix_nvt_sllod.html,
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can be used in conjunction with the SRD model.
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For more details on how the SRD model is implemented in LAMMPS, "this
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paper"_#Petersen describes the implementation and usage of pure SRD
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fluids. "This paper"_#Lechman, which is nearly complete, describes
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the implementation and usage of mixture systems (solute particles in
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an SRD fluid). See the examples/srd directory for sample input
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scripts using SRD particles in both settings.
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This fix does 2 things:
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(1) It advects the SRD particles, performing collisions between SRD
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and big particles or walls every timestep, imparting force and torque
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to the big particles. Collisions also change the position and
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velocity of SRD particles.
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(2) It resets the velocity distribution of SRD particles via random
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rotations every N timesteps.
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SRD particles have a mass, temperature, characteristic timestep
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dt_SRD, and mean free path between collisions (lamda). The
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fundamental equation relating these 4 quantities is
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lamda = dt_SRD * sqrt(Kboltz * Tsrd / mass) :pre
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The mass of SRD particles is set by the "mass"_mass.html command
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elsewhere in the input script. The SRD timestep dt_SRD is N times the
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step dt defined by the "timestep"_timestep.html command. Big
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particles move in the normal way via a time integration "fix"_fix.html
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with a short timestep dt. SRD particles advect with a large timestep
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dt_SRD >= dt.
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If the {lamda} keyword is not specified, the the SRD temperature
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{Tsrd} is used in the above formula to compute lamda. If the {lamda}
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keyword is specified, then the {Tsrd} setting is ignored and the above
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equation is used to compute the SRD temperature.
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The characteristic length scale for the SRD fluid is set by {hgrid}
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which is used to bin SRD particles for purposes of resetting their
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velocities. Normally hgrid is set to be 1/4 of the big particle
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diameter or smaller, to adequately resolve fluid properties around the
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big particles.
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Lamda cannot be smaller than 0.6 * hgrid, else an error is generated
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(unless the {shift} keyword is used, see below). The velocities of
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SRD particles are bounded by Vmax, which is set so that an SRD
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particle will not advect further than Dmax = 4*lamda in dt_SRD. This
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means that roughly speaking, Dmax should not be larger than a big
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particle diameter, else SRDs may pass thru big particles without
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colliding. A warning is generated if this is the case.
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Collisions between SRD particles and big particles or walls are
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modeled as a lightweight SRD point particle hitting a heavy big
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particle of given diameter or a wall at a point on its surface and
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bouncing off with a new velocity. The collision changes the momentum
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of the SRD particle. It imparts a force and torque to the big
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particle. It imparts a force to a wall. Static or moving SRD walls
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are setup via the "fix wall/srd"_fix_wall_srd.html command. For the
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remainder of this doc page, a collision of an SRD particle with a wall
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can be viewed as a collision with a big particle of infinite radius
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and mass.
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The {collision} keyword sets the style of collisions. The {slip}
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style means that the tangential component of the SRD particle momentum
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is preserved. Thus a force is imparted to a big particle, but no
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torque. The normal component of the new SRD velocity is sampled from
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a Gaussian distribution at temperature {Tsrd}.
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For the {noslip} style, both the normal and tangential components of
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the new SRD velocity are sampled from a Gaussian distribution at
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temperature {Tsrd}. Additionally, a new tangential direction for the
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SRD velocity is chosen randomly. This collision style imparts torque
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to a big particle. Thus a time integrator "fix"_fix.html that rotates
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the big particles appropriately should be used.
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:line
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The {overlap} keyword should be set to {yes} if two (or more) big
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particles can ever overlap. This depends on the pair potential
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interaction used for big-big interactions, or could be the case if
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multiple big particles are held together as rigid bodies via the "fix
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rigid"_fix_rigid.html command. If the {overlap} keyword is {no} and
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big particles do in fact overlap, then SRD/big collisions can generate
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an error if an SRD ends up inside two (or more) big particles at once.
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How this error is treated is determined by the {inside} keyword.
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Running with {overlap} set to {no} allows for faster collision
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checking, so it should only be set to {yes} if needed.
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The {inside} keyword determines how a collision is treated if the
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computation determines that the timestep started with the SRD particle
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already inside a big particle. If the setting is {error} then this
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generates an error message and LAMMPS stops. If the setting is {warn}
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then this generates a warning message and the code continues. If the
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setting is {ignore} then no message is generated. One of the output
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quantities logged by the fix (see below) tallies the number of such
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events, so it can be monitored. Note that once an SRD particle is
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inside a big particle, it may remain there for several steps until it
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drifts outside the big particle.
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The {exact} keyword determines how accurately collisions are computed.
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A setting of {yes} computes the time and position of each collision as
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SRD and big particles move together. A setting of {no} estimates the
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position of each collision based on the end-of-timestep positions of
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the SRD and big particle. If {overlap} is set to yes, the setting of
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the {exact} keyword is ignored since time-accurate collisions are
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needed.
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The {radius} keyword scales the effective size of big particles. If
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big particles will overlap as they undergo dynamics, then this keyword
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can be used to scale down their effective collision radius by an
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amount {rfactor}, so that SRD particle will only collide with one big
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particle at a time. For example, in a Lennard-Jones system at a
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temperature of 1.0 (in reduced LJ units), the minimum separation
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bewteen two big particles is as small as about 0.88 sigma. Thus an
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{rfactor} value of 0.85 should prevent dual collisions.
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The {bounce} keyword can be used to limit the maximum number of
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collisions an SRD particle undergoes in a single timestep as it
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bounces between nearby big particles. Note that if the limit is
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reached, the SRD can be left inside a big particle. A setting of 0 is
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the same as no limit.
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:line
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There are 2 kinds of bins created and maintained when running an SRD
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simulation. The first are "SRD bins" which are used to bin SRD
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particles and reset their velocities, as discussed above. The second
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are "search bins" which are used to identify SRD/big particle
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collisions.
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The {search} keyword can be used to choose a search bin size for
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identifying SRD/big particle collisions. The default is to use the
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{hgrid} parameter for SRD bins as the search bin size. Choosing a
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smaller or large value may be more efficient, depending on the
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problem. But, in a statistical sense, it should not change the
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simulation results.
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The {cubic} keyword can be used to generate an error or warning when
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the bin size chosen by LAMMPS creates SRD bins that are non-cubic or
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different than the requested value of {hgrid} by a specified
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{tolerance}. Note that using non-cubic SRD bins can lead to
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undetermined behavior when rotating the velocities of SRD particles,
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hence LAMMPS tries to protect you from this problem.
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LAMMPS attempts to set the SRD bin size to exactly {hgrid}. However,
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there must be an integer number of bins in each dimension of the
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simulation box. Thus the actual bin size will depend on the size and
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shape of the overall simulation box. The actual bin size is printed
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as part of the SRD output when a simulation begins.
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If the actual bin size in non-cubic by an amount exceeding the
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tolerance, an error or warning is printed, depending on the style of
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the {cubic} keyword. Likewise, if the actual bin size differs from
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the requested {hgrid} value by an amount exceeding the tolerance, then
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an error or warning is printed. The {tolerance} is a fractional
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difference. E.g. a tolerance setting of 0.01 on the shape means that
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if the ratio of any 2 bin dimensions exceeds (1 +/- tolerance) then an
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error or warning is generated. Similarly, if the ratio of any bin
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dimension with {hgrid} exceeds (1 +/- tolerance), then an error or
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warning is generated.
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IMPORTANT NOTE: The fix srd command can be used with simluations the
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size and/or shape of the simulation box changes. This can be due to
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non-periodic boundary conditions or the use of fixes such as the "fix
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deform"_fix_deform.html or "fix wall/srd"_fix_wall_srd.html commands
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to impose a shear on an SRD fluid or an interaction with an external
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wall. If the box size changes then the size of SRD bins must be
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recalculated every reneighboring. This is not necessary if only the
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box shape changes. This re-binning is always done so as to fit an
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integer number of bins in the current box dimension, whether it be a
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fixed, shrink-wrapped, or periodic boundary, as set by the
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"boundary"_boundary.html command. If the box size or shape changes,
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then the size of the search bins must be recalculated avery
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reneighboring. Note that changing the SRD bin size may alter the
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properties of the SRD fluid, such as its viscosity.
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The {shift} keyword determines whether the coordinates of SRD
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particles are randomly shifted when binned for purposes of rotating
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their velocities. When no shifting is performed, SRD particles are
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binned and the velocity distribution of the set of SRD particles in
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each bin is adjusted via a rotation operator. This is a statistically
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valid operation if SRD particles move sufficiently far between
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successive rotations. This is determined by their mean-free path
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lamda. If lamda is less than 0.6 of the SRD bin size, then shifting
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is required. A shift means that all of the SRD particles are shifted
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by a vector whose coordinates are chosen randomly in the range \[-1/2
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bin size, 1/2 bin size\]. Note that all particles are shifted by the
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same vector. The specified random number seed is used to generate
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these vectors. This operation sufficiently randomizes which SRD
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particles are in the same bin, even if lamda is small.
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If the {shift} style is set to {no}, then no shifting is performed,
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but bin data will be communicated if bins overlap processor
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boundaries. An error will be generated if lamda < 0.6 of the SRD bin
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size. If the {shift} style is set to {possible}, then shifting is
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performed only if lamda < 0.6 of the SRD bin size. A warning is
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generated to let you know this is occurring. If the {shift} style is
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set to {yes} then shifting is performed regardless of the magnitude of
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lamda.
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The shift seed is not used if the {shift} style is set to {no}, but
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must still be specified.
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Note that shifting of SRD coordinates requires extra communication,
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hence it should not normally be enabled unless required.
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The {tstat} keyword will thermostat the SRD particles to the specified
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{Tsrd}. This is done every N timesteps, during the velocity rotation
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operation, by rescaling the thermal velocity of particles in each SRD
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bin to the desired temperature. If there is a streaming velocity
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associated with the system, e.g. due to use of the "fix
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deform"_fix_deform.html command to perform a simulation undergoing
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shear, then that is also accounted for. The mean velocity of each bin
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of SRD particles is set to the position-dependent streaming velocity,
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based on the coordinates of the center of the SRD bin. Note that
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collisions of SRD particles with big particles or walls has a
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thermostatting effect on the colliding particles, so it may not be
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necessary to thermostat the SRD particles on a bin by bin basis in
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that case. Also note that for streaming simulations, if no
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thermostatting is performed (the default), then it may take a long
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time for the SRD fluid to come to equilibrium with a velocity profile
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that matches the simulation box deformation.
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The {rescale} keyword enables rescaling of an SRD particle's velocity
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if it would travel more than 4 mean-free paths in an SRD timestep. If
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an SRD particle exceeds this velocity it is possible it will be lost
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when migrating to other processors or that collisions with big
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particles will be missed, either of which will generate errors. Thus
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the safest mode is to run with rescaling enabled. However rescaling
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removes kinetic energy from the system (the particle's velocity is
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reduced). The latter will not typically be a problem if
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thermostatting is enabled via the {tstat} keyword or if SRD collisions
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with big particles or walls effectively thermostat the system. If you
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wish to turn off rescaling (on is the default), e.g. for a pure SRD
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system with no thermostatting so that the temperature does not decline
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over time, the {rescale} keyword can be used. The {no} value turns
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rescaling off during collisions and the per-bin velocity rotation
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operation. The {collide} and {rotate} values turn it on for
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one of the operations and off for the other.
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:line
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IMPORTANT NOTE: This fix is normally used for simulations with a huge
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number of SRD particles relative to the number of big particles,
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e.g. 100 to 1. In this scenario, computations that involve only big
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particles (neighbor list creation, communication, time integration)
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can slow down dramatically due to the large number of background SRD
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particles.
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Three other input script commands will largely overcome this effect,
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speeding up an SRD simulation by a significant amount. These are the
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"atom_modify first"_atom_modify.html, "neigh_modify
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include"_neigh_modify.html, and "communicate group"_communicate.html
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commands. Each takes a group-ID as an argument, which in this case
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should be the group-ID of the big solute particles.
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Additionally, when a "pair_style"_pair_style.html for big/big particle
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interactions is specified, the "pair_coeff"_pair_coeff.html command
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should be used to turn off big/SRD interactions, e.g. by setting their
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epsilon or cutoff length to 0.0.
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The "delete_atoms overlap" command may be useful in setting up an SRD
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simulation to insure there are no initial overlaps between big and SRD
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particles.
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:line
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[Restart, fix_modify, output, run start/stop, minimize info:]
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No information about this fix is written to "binary restart
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files"_restart.html. None of the "fix_modify"_fix_modify.html options
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are relevant to this fix.
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This fix tabulates several SRD statistics which are stored in a vector
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of length 12, which can be accessed by various "output
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commands"_Section_howto.html#howto_15. The vector values calculated
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by this fix are "intensive", meaning they do not scale with the size
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of the simulation. Technically, the first 8 do scale with the size of
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the simulation, but treating them as intensive means they are not
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scaled when printed as part of thermodyanmic output.
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These are the 12 quantities. All are values for the current timestep,
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except for quantity 5 and the last three, each of which are
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cummulative quantities since the beginning of the run.
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(1) # of SRD/big collision checks performed
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(2) # of SRDs which had a collision
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(3) # of SRD/big colllisions (including multiple bounces)
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(4) # of SRD particles inside a big particle
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(5) # of SRD particles whose velocity was rescaled to be < Vmax
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(6) # of bins for collision searching
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(7) # of bins for SRD velocity rotation
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(8) # of bins in which SRD temperature was computed
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(9) SRD temperature
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(10) # of SRD particles which have undergone max # of bounces
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(11) max # of bounces any SRD particle has had in a single step
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(12) # of reneighborings due to SRD particles moving too far :ul
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No parameter of this fix can be used with the {start/stop} keywords of
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the "run"_run.html command. This fix is not invoked during "energy
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minimization"_minimize.html.
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[Restrictions:]
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This command can only be used if LAMMPS was built with the SRD
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package. See the "Making LAMMPS"_Section_start.html#start_3 section
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for more info on packages.
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[Related commands:]
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"fix wall/srd"_fix_wall_srd.html
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[Default:]
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The option defaults are lamda inferred from Tsrd, collision = noslip,
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overlap = no, inside = error, exact = yes, radius = 1.0, bounce = 0,
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search = hgrid, cubic = error 0.01, shift = no, tstat = no, and
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rescale = yes.
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:line
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:link(Hecht)
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[(Hecht)] Hecht, Harting, Ihle, Herrmann, Phys Rev E, 72, 011408 (2005).
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:link(Petersen)
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[(Petersen)] Petersen, Lechman, Plimpton, Grest, in' t Veld, Schunk, J
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Chem Phys, 132, 174106 (2010).
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:link(Lechman)
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[(Lechman)] Lechman, et al, in preparation (2010).
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